Antiphospholipid antibodies are a heterogenous family of antibodies associated with an increased risk for venous and arterial thrombosis, recurrent fetal loss, and thrombocytopenia. Several kindreds have been described with a familial form of the antiphospholipid antibody syndrome, confirming an inherited tendency to antibody formation. In these patients, autoantibodies that bind to proteins involved in normal hemostasis (e.g., prothrombin) are associated with an increased thrombotic risk. Antiphospholipid antibodies are also frequently seen in patients with vascular grafts who sustain thrombotic occlusions. Bovine thrombin is commonly used during vascular surgery, and we have shown that most patients exposed to bovine thrombin during cardiovascular surgery develop elevated antibody levels to several bovine and human coagulation proteins, as well as elevated antiphospholipid antibody levels. We hypothesize that different groups of patients with antiphospholipid antibodies and thrombotic complications share a common clinical phenotype that has developed in response to distinct etiologic mechanisms. To probe the pathophysiologic mechanisms of these antibodies, we propose to investigate two prothrombotic antibody subsets frequently identified in patients with autoantibodies by the following aims. First, we will define the molecular interactions of antiprothrombin autoantibodies with prothrombin and determine the functional and clinical significance of these autoantibodies. Epitopes recognized by antiprothrombin autoantibodies will be identified. Functional consequences of antibody binding to prothrombin will be determined and correlated to thrombotic events. Second, we will investigate the mechanisms whereby antiphospholipid autoantibodies interfere with the anticoagulant mechanisms of activated protein C. The functional consequences of antiphospholipid autoantibodies on the inactivation of factor Va by the activated protein C complex will be characterized and correlated to clinical outcome. Third, antiphospholipid antibodies that develop in patients exposed to bovine thrombin will be compared to antiphospholipid autoantibodies for antiprothrombin activity as well as inhibition of the activated protein C complex. These studies will improve our understanding of the role(s) of distinct antibody subsets in producing a hypercoagulable state in different patient subsets with antiphospholipid antibody syndromes.